PX157 - D4 - transformer

windings in the primary and the secondary coils are in opposite directions
the magnetic flux is the same for both sides
faraday's law:

\begin{aligned} ϵ_{S} & = - N_{S} \frac{d ϕ_{B}}{d t} \\ ϵ_{P} & = - N_{P} \frac{d ϕ_{B}}{d t} \\ \frac{ϵ_{S}}{N_{S}} & = \frac{ϵ_{P}}{N_{P}} \end{aligned}

for $N_{S} > N_{P} ⟹ ϵ_{S} > ϵ_{P} :$ step-up transformer
for $N_{S} < N_{P} ⟹ ϵ_{S} < ϵ_{P} :$ step-down transformer
from conservation of energy: $P = I V$ is conserved:

I_{S} ϵ_{S} = I_{P} ϵ_{P} ⟹ N_{S} I_{S} = N_{P} I_{P}